This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Heat transfer labels are multilayered laminates, with each layer having its own function. For example, heat transfer labels generally include an adhesive layer, an ink design layer, and a release layer. The release layer may be a wax release layer, and is often directly adjacent a carrier sheet, such as on a roll or web of labels. Thus, in such an example, the label may be thought to include a “support portion” (e.g., carrier sheet and release layer) and a “transfer portion” (e.g., ink design layer and adhesive layer). When subjected to heat, the wax release layer softens or melts, thereby allowing the transfer portion to be separated from the carrier sheet, and the adhesive layer adheres the ink design layer to an article being labeled. Alternatively, all or part of the wax release layer may transfer as well, to provide protection to the ink design layer. Additionally or alternatively, the labels may include a separate protective layer overlying the ink design layer to protect the ink design layer from abrasion and product resistance.
More specifically, in the heat transfer labeling process, the label-carrying sheet is subjected to heat, and the label is pressed onto an article with the adhesive layer making direct contact with the article. (Alternatively, in an embodiment having an adhesive incorporated in the ink design layer, the ink design layer may make direct contact with the article.) As the paper sheet is subjected to heat, the wax layer begins to soften or melt so that the paper sheet can be released from the ink design layer. (And, as described above, a portion of the wax layer may be transferred with the ink design layer and a portion of the wax layer may remain with the paper sheet.) After transfer of the ink design layer to the article, the paper sheet is removed, leaving the ink design layer firmly affixed to the article. In an alternate embodiment, where the wax layer also transfers, the wax layer thus may serve two purposes: (1) to provide release of the ink design layer from the sheet upon application of heat to the sheet, and (2) to form a protective layer over the transferred ink design layer. After transfer of the label to the article, the transferred wax release layer may be subjected to a postflaming technique which enhances the optical clarity of the layer (thereby enabling the ink design layer therebeneath to be better observed) and which enhances the protective properties of the transferred wax layer.
Such heat transfer labels have been used to decorate a variety of articles, such as polyethylene, polypropylene, PET, and acrylonitrile articles. For example, such articles may include high-density polyethylene (HDPE) containers, low-density polyethylene (LDPE) containers, and polypropylene containers. One example of a heat transfer label that has been used to decorate polyethylene (PE) containers includes a paper carrier sheet overcoated with a wax release layer (approximately 6-8 lbs. wax/3,000 square feet of paper carrier web). A number of layers that make up the transfer portion of the heat transfer layer are then associated with the paper carrier sheet with wax release layer of the support portion. In one exemplary embodiment of a current heat transfer label, these layers may include a protective lacquer layer, an ink design layer, and an adhesive layer. The protective lacquer layer is printed on the wax release layer. The ink design layer is printed on the protective lacquer layer. And the adhesive layer is printed on the ink design layer. Those skilled in the art will realize that these layers, number of layers, and configuration of layers is merely exemplary of one heat transfer label presently in use.
Heat transfer labels are generally made using rotogravure printing techniques. In rotogravure techniques, the printing plates for the ink, lacquer, adhesive, and/or any other component(s) are in cylinder form, and include wells that are etched or engraved to differing depths and/or sizes to provide the image or images. The component, such as ink, lacquer, or adhesive, is applied directly to the cylinders, such as by rotating them in baths containing the component so that each cell of the cylinder is flooded with the component. A doctor blade wipes away the excess component, and capillary action of the substrate and pressure from impression rollers draw the component out of the wells and transfer it to the substrate. However, there are drawbacks to using rotogravure printing. Problems with this method of preparing such labels include the fact that the cost of tooling to produce the labels (i.e., the cost of preparing engraved cylinders) is very expensive. And thus, long production runs are required to make the manufacturing process efficient.
Further, heat transfer labels are printed with solvent inks because the solvent ink chemistry is flexible and able to withstand significant stretch during the decoration step. For example, once the label is printed on the wax carrier, the label is applied to an article. During this decoration step, it may be preferable to “stretch” the label (i.e., the surface speed of the article exceeds the speed of the moving web). This can help to prevent vertical gathers (also called creping) that may be observed due to the mechanical nature of the decorating process. The ability to stretch the label due to the solvent-based components is also beneficial in that it allows for the decoration of tapered containers (such as cups, buckets) and objects with compound curves.
However, the solvent-based components that are used when making heat transfer labels pose a number of drawbacks. In particular, such solvent-based components raise health, safety, and environmental concerns. For example, most organic solvents are flammable or highly flammable, depending on their volatility. Mixtures of solvent vapors and air can explode. Solvent vapors are heavier than air, and so they can and will sink and can travel large distances in an undiluted or nearly undiluted state. Solvent vapors can also be found in supposedly empty drums and cans, posing a flash fire hazard. Further, many solvents can lead to a sudden loss of consciousness if inhaled in large amounts. And, some solvents pose health issues—such as chloroform and benzene, which are carcinogenic. Many others can damage internal organs like the liver, the kidneys, or the brain. More solvent-related health issues arise from spills or leaks of solvents that reach the underlying soil. Since solvents readily migrate substantial distances, the creation of widespread soil contamination is not uncommon. The problems of subsurface solvent contamination can be greatly amplified if the solvents leach into, or otherwise gain access to, water supply. These problems also result in many difficulties, and in high costs, of using solvent-based materials and disposing of solvents or solvent waste during and following production runs.
The use of flexographic printing can reduce the costs associated with the more expensive gravure printing, especially as run lengths get shorter. One reason that flexographic printing has not been pursued aggressively until recently is that the chemistry of the solvent inks are incompatible with the photopolymer plates due to a phenomenon known as “swelling.” Basically, when certain traditional rotogravure printing solvent components are applied to the photopolymer plate used in flexographic printing, the components can degrade and be absorbed by the plate. Thus, the plates can only be used for a single run. Thereafter, a new plate would have to be used. Such a rapid turnover in plates is extremely expensive to the point that it is not feasible.
Thus, the desire in the printing and label industry is to move away from solvent-based technology. The present movement in the industry has been from solvent-based technology to water-based technology. However, water-based technology has its own limitations. For example, water-based ink has lower product resistance. In addition, ink quality (pH, drying, and viscosity) must be diligently monitored and maintained throughout the print run to avoid variation in print quality. And, heat transfer products produced with water based ink are lower gloss, which is not desirable.
UV-curable inks are not solvent-based. And so, use of such UV-curable inks could eliminate the drawbacks described above with solvent-based components. However, there are additional drawbacks that arise when UV-curable inks are used to prepare labels such as heat transfer labels. For example, UV-curable ink layers do not exhibit the ability to “stretch” like the solvent-based and water-based components, and so heat transfer labels prepared with UV-curable inks will “crack.” This cracking is a result of the cross-linking that occurs when the UV-curable inks are cured by exposure to UV radiation. As is well known to those skilled in the art, cross-links are bonds that link one polymer chain to another. When polymer chains are linked together by cross-links, they lose some of their ability to move as individual polymer chains. In other words, the present UV inks that are used in labeling processes are thermoset materials. Thus, the curing process transforms the resins and other components of the UV-curable inks by cross-linking into a rigid structure. Then, as the label including this rigid ink structure is applied to an article (such as a bottle), the label must stretch during its release from any carrier sheet and during application to the contour of the article. While flowable resins (i.e., thermoplastic materials) can move in such fashion, the rigid nature of the thermoset UV inks cannot, and so the UV inks tend to “break,” due to the cross-links, which causes a cracking phenomenon to be exhibited in the label. This cracking results in labels that are not aesthetically pleasing, and thus are not useful to a label manufacturer or label customer.